In thermal ink jet printing, droplets of ink are selectively emitted from a plurality of drop ejectors in a printhead, in accordance with digital instructions, to create a desired image on a surface. The printhead typically comprises a linear array of ejectors for conveying the ink to the sheet. The printhead may move back and forth relative to a surface, for example to print characters, or the linear array may extend across the entire width of a sheet (e.g. a sheet of plain paper) moving relative to the printhead. The ejectors typically comprise capillary channels, or other ink passageways, forming nozzles which are connected to one or more common ink supply manifolds. Ink from the manifold is retained within each channel until, in response to an appropriate digital signal, the ink in the channel is rapidly heated and vaporized by a heating element disposed within the channel. This rapid vaporization of the ink creates a bubble which causes a quantity of ink to be ejected through the nozzle to the sheet.
U.S. Pat. No. 5,223,853, entitled "Electronic Spot Size Control in a Thermal Ink Jet Printer," discloses a system by which ink jet droplets of a consistent size may be ejected from a thermal ink jet printhead, regardless of the original temperature of the liquid ink in the printhead. As described in detail in that patent, one of the most crucial parameters for image quality in an ink jet printer is the spot size of individual droplets of ink emitted from the printhead, and in turn the most crucial parameter for determining the spot size of individual droplets is the temperature of the liquid ink immediately before ejection. The system of U.S. Pat. No. 5,223,853 operates on the principle of first measuring the temperature of the liquid ink in the printhead and then, in response to this measured temperature, providing to the printhead an optimal combination of power (typically voltage) and pulse duration to a heating element to vaporize the liquid ink and cause it to be ejected from the printhead. From the standpoints of preventing overheating of the printhead chip, avoiding kogation of ink within the printhead, and other practical concerns, a selected pulse duration must be coupled with an appropriate power level, and vice-versa. For each measured temperature there is provided in this system a best combination of power and pulse width, as opposed to a system which merely increases or decreases one input or the other.
Although the temperature of the liquid ink has a demonstrable relationship with the size of an individual droplet of ink on the print sheet, and therefore of the general print quality, several other specific conditions will have a noticeable effect as well. Foremost among these is the particular character of the print sheet itself: for example, different types of paper may have substantially different types of absorbency or gloss, which will directly affect the character of images printed thereon, and these particular qualities of the sheet will in turn vary somewhat with ambient conditions, particularly humidity. In general, a more absorbent type of paper, such as newsprint, is very good at holding ink ejected thereon--although, at the same time, particularly absorbent types of paper may induce "wicking" of ink along the fibers thereof, causing a blurry image. At the other extreme, a coated transparency has very small absorbency. Because any absorbent member, such as a piece of paper, has a finite ability to hold liquid, a sheet which is already somewhat damp from the ambient humidity will be less able to hold ink from the printing process. There may indeed be even more variables affecting the spot size and print quality, some of which may be imperfectly understood. Still, the only major parameter affecting print quality which is likely to change in the course of printing is the temperature of the liquid ink: the action of the heating elements within a printhead causes a substantial increase in temperature of the printhead in the course of printing a job.
In the prior art, U.S. Pat. No. 5,189,521 discloses a control system useful for balancing the densities of a plurality of different-colored ink-jet heads. The system includes the steps of creating a test pattern with the plurality of ink-jet heads, and reading the test pattern to yield a "density distribution" indicative of the condition of the various heads.
U.S. Pat. No. 5,066,961 discloses a thermal (as opposed to init-jet) printer including a system for compensating for heat accumulated while the printer is in use, to yield a constant image density. The system predicts, based on the incoming image data, the temperature of the thermal head after energy is applied to the thermal head, and then the pulse width is adjusted accordingly.
U.S. Pat. No. 5,038,208 discloses an ink-jet copier in which data corresponding to the characteristics of the particular printheads is stored in a memory, and the image-forming signals are modified in view of this data.